Cyclic carbonates are an important raw material for engineering plastics like polycarbonates. Currently, the latter are manufactured using phosgene, a highly toxic, irritating and corrosive gas, inhalation of which can cause fatal respiratory damage. The total demand of polycarbonates is more than 1.2 million tons per annum. The demand for polycarbonates is expected to increase by approximately 9% per year. Plastics of this material are widely used in electric and electronic industry, building industry, optical data storage media, automotive industry, package industry, headlamp diffuser lens and bottles for water and milk. Polycarbonates of aliphatic type are used as plasticizers, stabilizers for vinyl chloride polymers, co-monomers in polyurethane synthesis, lubricants, elastomers (functionalized PC with pendent vinyl group) and biodegradable and biomedical materials for drug delivery. Polycarbonates are commercially prepared by condensation of 4-hydroxydiphenylbutane and phosgene (COCl2) in the presence of substituted amines and alkali (Encyclopedia of Chemical Processing and Design, Vol 40, Ed. by J. J. McKetta and W. A. Cunningham, Marcel Dekker Inc., New York, 1992, p. 136 and Ulmann's encyclopedia of Industrial Chemistry, Vol. A 21, Ed. by B. Elvers, S. Hawkins and G. Schulz, 5th ed. VCH Verlagsgesellschaft, GmbH, Germany 1992, p. 207). However, this method of preparation is highly toxic and dangerous. Eco-friendly routes for the preparation of polycarbonates or their precursor cyclic carbonates are highly desirable.
Inoue et al. (J. Poly. Sci. Polym. Lett. Vol. 7, pp. 298 (1969)) reported for the first time that polycarbonates can be prepared by cyclo-addition of epoxides to carbon dioxide in the presence of zinc catalyst. However, this reaction usually requires high temperatures and pressures. Homogeneous catalysts like CH3SnBr3, Ph4SbBr and n-Bu3SnI were reported to be effective at low temperatures, but high concentration of the catalyst (≧1 mol %) is required (Matsuda et al., Chem. Lett. (1979) 573; Nomura et al., J. Org. Chem. 45 (1980) 3735; Baba et al., Bull. Chem. Soc. Jpn 60 (1987) 1552). This necessitates expensive catalyst separation and product purification. Organometallic complexes of Zn and the complexes of low-valent transition metals e.g., Ni(0) and Cu(I) were reported to exhibit higher catalytic activity but they are highly sensitive to air and moisture. Phthalocyanine and Schiff base complexes showed good activity for the cyclo-addition reaction (Ji et al., Appl. Catal. A: General 203 (2000) 329). Again, the catalyst separation is a major problem.
Exxon Research & Engineering Co. (U.S. Pat. No. 4,824,969) has developed a process for preparing cyclic carbonate esters from olefins in a single reaction mixture using osmium compound, copper containing co-catalyst I (e.g., CuBr2), co-catalyst II (e.g., pyridine) and water. Shell Oil Company (U.S. Pat. Nos. 4,826,887 and 4,826,953) reported the process for the preparation of polycarbonates in the presence of catalytic amounts of a double metal cyanine complex and (a) one or more salts composed of at least bivalent metal ions and metal-free anions having a solubility in water of at least 1 g/100 ml and one or more no-metal containing acids. U.S. Pat. No. 6,469,193 reports the preparation of aliphatic carbonates from aliphatic alcohols, alkyl halides and carbon dioxide in the presence of cesium carbonate and tetrabutyl ammonium iodide. U.S. Pat. No. 6,407,264 reports a process involving the reaction of alkylene oxide with carbon dioxide in the presence of a catalyst system comprising of a metal halide and pyridine or pyridine derivative. U.S. Pat Nos. 6,399,536, 5,391,767 and 6,288,202 and UK Pat Appl. GB 2352449 A1, PCT Int. Appl. WO 2000008088 A1, Ger. Offen. DE 19737547 A1 and Eur. Pat. Appl. EP 864361 A2 are all related to this process. However, in all the prior art processes the reaction is in homogeneous medium, high catalyst amounts are needed which necessitates elaborate process as an additional step for catalyst separation/recycle. There are a few reports on the use of solid catalysts like silica supported guanidine (Barbarini et al Tetrahedron Lett. 44 (2003) 2931) and MCM-supported phthalocyanine (Lu et al., J. Mol. Catal. A: Chemical 186 (2002) 33) complexes for this reaction, however larger amounts catalyst and long reaction times (>15 h) are required for high yield of cyclic carbonate.